Fedora core 6 live is very slow, very sluggish in comparison to Knoppix 5.1 - In the same machine, Knoppix 5.1 boots doubly fast, and runs equally fast.

Fedora core 6 live does not offer many boot options as well. Whereas, Knoppix 5.1 gives you more than a dozen boot options - including to boot from RAM, Hard-disk-copied image and many more.

Knoppix 5.1 has built in full multimedia support. Whereas, you need to install mp3 codec in Fedora core 6 live additionally to play your favorite MP3 files.

Knoppix 5.1 has KDE, whereas Fedora Live is built upon Gnome. KDE has much more applications included and hence Knoppix.

However, Knoppix 5.1 falls behind Fedora core 6 Live CD on Language section. Knoppix 5.1 had about a dozen western language support in all, and you need to install additional language pack for additional language such as Hindi (Indic). Fedora core 6 Live CD contains all the language packs available for Linux distribution that main Fedora tree carries. It means, you will get your language environment in Fedora Core 6 Live CD if it is available in Fedora Core 6 Main distribution. Language packs, fonts, Keyboard, everything you need for your language environment will be available to you when you login to that language environment. Great!

Final Verdict: If you want out of the box support for your particular language in your Linux machine, go for Fedora core 6 Live CD. Else, Knoppix 5.1 is the best bet indeed.